In this interview, Yoshida casually confirms the existence of unannounced elements in the racing sim, including a track editor and go-karts. For every individual driver, Yoshida says there is " a driving experience to suit, whether that is taking on the Top Gear test track, chucking cars around stunt arenas, go-karting, creating and sharing your own track or intense head-to-head races."
Elsewhere in the interview (and just as nonchalantly), Yoshida references some " new game-enhancing graphical effects, such as damage, night racing and weather." Damage has already been confirmed and weather rumored.
Of course, we aren't totally sure Gran Turismo 5 is a real game yet, and if it is as imaginary as it seems to be, you can say pretty much anything is in it, like: "Gran Turismo 5 will feature Crocodile Mile racing." See? There's as much proof for that feature that we just made up as there is for anything else, and that will be the case until the game is released.